Opa proteins from Neisseria gonorrhoeae and N. meningitidis pose an intriguing biological riddle: How can a structure that is tolerant of sequence diversity bind to the same family of receptors, and also provide receptor selectivity among the receptor variants? Opa is a class of outer membrane proteins that bind to various host receptors that induce engulfment of the bacterium. The Opa receptor specificity is predominantly determined by two segments (hypervariable, HV, 1 and 2) in extracellular loops 2 and 3. The Opa barrel sequences exhibit 75% identity, but these loop regions display high sequence diversity. A structural motif for receptor engagement and selectivity has not been identified. Our overarching goal is to determine how Opa proteins interact with their cognate human receptors in order to decipher the mechanism of the molecular recognition and the specific molecular determinants of several Opa-receptor interactions. Knowledge of the structure, dynamics, and specific interactions of Opa proteins and receptors will be used to design targeted liposome delivery to human cells. We have only begun to unravel the answers to this complex puzzle. Our recent Opa60 structure and dynamics data indicate the extracellular loops are disordered, yet sample a restricted volume such that frequent short-lived transient interactions occur between the loops on the nanosecond timescale. These results partly answer the riddle - the structure is not discrete or well-defined, but rather can tolerate sequence variability in the HV sequences as a mechanism for binding. Molecular dynamics simulations indicate the loops engage in transient interactions, specifically between the HV1 and HV2 regions; this may be a mechanism by which the protein can sample conformers in which HV1 and HV2 are in proximity and can bind receptor. Though this protein system may be inherently ill-defined in terms of the typical structural biology paradigm, we speculate that a specific, well-defined mechanism is at play. We propose a hypothesis wherein the clustering of hydrophobic residues mediates transient interactions between HV1 and HV2. Once localized through these interactions, we hypothesize further that two or more states are sampled on the s - ms time scale with a sub-set of conformations competent for receptor binding. To test these hypotheses, we will use a hybrid method approach to determine the dynamics and interactions of Opa60 in the unbound and receptor bound states. In addition, we have developed in vivo assays that tests the biological significance of the Opa- receptor molecular determinants that we elucidate with in vitro studies. The combined results of the proposed studies will determine how Opa proteins interact with their cognate human receptors providing insights into the pathogenesis and potential treatments for N. gonorrhoeae and N. meningitides, of which the former has reached super bug status because of antibiotic resistance. Longer term, this knowledge will serve as a platform for designing a delivery system that targets foreign entities into specific subsets of human cells.